In the past, asbestos yarn was the only binding yarn to be used with incandescent bodies. This yarn was enduring at high temperatures and could be easily knotted. It however has the disadvantageous characteristic that it is damaging to health and has therefore been prohibited in many countries. The disadvantageous properties of asbestos, in particular the limited usability on account of its prohibition has led to the search and application of new binding yarns.
Thus it has been attempted to use cotton yarn which is impregnated in metal salts, e.g. of rare earths. During the burning-away of the incandescent body, the cotton burns without any residue and an oxide skeleton in the form of a yarn remains. The skeleton is likewise enduring at high temperatures and in the original condition the yarn may be easily knotted. The disadvantageous property lies in the fact that after the burning-away of the incandescent body only the oxide skeleton arising from the impregnation salts remains, which has no tear strength and may not hold the incandescent body on the burner for a long time. This yarn therefore was hardly used in practice.
Binding yarns based on glass fiber, ceramics, organic materials, e.g. aramide, carbon fibers etc. are also known. The endurance at high temperature is counted amongst the advantageous properties. However, at the same time there are also disadvantages. These yarns have such a smooth and such a friction-free surface that they are difficult to knot or the knot easily becomes undone. (A knot only holds on account of the friction). With a low Tex number there is a low tear strength, poor processing ability, e.g. on sewing, cutting, etc.
Common binding yarns from which the invention proceeds consist of a glass fiber or ceramic core or a silica core of approx. 0.2 mm diameter enveloped with cotton of stack fibers according to the International patent application PCT/GB92/00432 so that there results a total diameter of approx. 0.5 mm.
These have a silica yarn core enduring at high temperatures and may be knotted since the smooth silica yarn is sheathed to excess with non-smooth cotton yarn, which overcomes the smoothness and renders the knot lasting. They are tear-resistant and may be easily processed. The disadvantage with these is the fact that the incandescent body which is originally rigidly attached to the burner, after the burning-away of the incandescent body, is seated loosely on the burner, by which means the mechanical durability on the burner is reduced. The initially tight knot is likewise loose after the burning-away of the wrapped winding. The manufacture is effected in an expensive DREF machine and the yarn is therefore very expensive. Furthermore, it is often the case that as a result of operational trouble the core is no longer co-conveyed, but the production continues to run. Such errors may not however be recognized by optical control so that after burning-away the binding yarn has disappeared and the incandescent body falls down.
Finally instead of a binding yarn, a round metal clip according to U.S. Pat. No. 5,116,220 and U.S. Pat. No. 5,639,231 are used. The metal clip is sewn into the incandescent body at the height at which the binding yarn would have been sewn in. This metal clip is enduring at high temperature and is operationally friendly since the user does not need to tie any yarn. The connection between the incandescent body and the burner does not have the same compactness, as is the case with the binding yarn. The incandescent body does not perfectly contact the burner (see U.S. Pat. No. 5,639,231). The lack of retention of the incandescent body on the burner is the price to pay for the operational friendliness of the metal clip.
It is therefore the object of the invention to provide an operationally friendly yarn which may be easily knotted and furthermore increases the application duration and durability of an incandescent hood subjected to knocks and impacts.